Conventional grain sorting machines have a rough grain sorting plate provided horizontally at the front and rear sides or with the front part raised and in a laterally sloped position. Sorting plates of this type are vibrated so as to shake up the grains at an average oscillation angle larger than the slope elevation angle. In other grain sorting machines air is blown through holes in the grain sorting plate without vibrating. In operation, a mixture of unhulled rice and unpolished rice is, for example, supplied onto said sorting plate, and different kinds of grain are collected at the front or rear side of said sorting plate, the different kinds of grains being separated by a separation wall provided beside said sorting plate. Finally, the grain is caused to flow out in a lateral direction. The distribution of grains on the sorting plate varies according to the amount and quality of mixed grain supplied. In addition, the greater the slop elevation angle of the sorting plate, the more the grain drifts to the rear part, and the greater the average oscillation angle, number of vibrations, or amplitude, the more the grain drifts to the front.
Accordingly, since the boundary of the different kinds of sorted grains e.g., unhulled rice, mixture of unhulled rice and unpolished rice, and unpolished rice, moves on the sorting plate, the separating wall for separating these different kinds of grains must be moved along an edge of the sorting plate.
As shown in Laying-open Pat. No. 51-47651, it has been the conventional practice to manually move the sorting wall along an edge of the sorting plate when the boundary of the different kinds of grains no longer coincides with the previous boundaries on the sorting plate, by observing the state of grain distribution on the sorting plate.
However, it is difficult to discern the boundary between unpolished grain and a mixture of unpolished grain and unhulled grain. If the boundary moves frequently, moving the separation wall is very troublesome. In particular, in the case of an oscillating type grain sorting machine, the machine must be stopped each time the separation wall is to be moved. Because of these disadvantages, work efficiency is very low.